If you have decided to make your pond from concrete then you will need to seal the top layer of render to stop the water getting in and the lime, which is harmful to fish, getting out.
The best way to do this is with G4 Sealer. You don't need a primer; just paint it straight onto the concrete. You can also use G4 to coat flagstones, waterfalls, statues and other concrete products to make them safe for your pond.
Clear or Black colour sealant available
For a black finish you need two coats of clear and two coats of black. Thats 440g per square metre of clear and 400g per square metre of colour.
